Our general hypothesis is that transplanted liver cells will survive and function in the context of a permissive microenvironment, best exemplified by the liver. A variety of interactions between microenvironmental factors, other types of liver cells, and soluble factors could regulate engraftment of transplanted cells in the liver. Also, cell therapy requires creation of an adequate mass of transplanted cells in the liver. To demonstrate whether transplanted cells can engraft and proliferate in specific disorders, studies in suitable animal models are necessary. Such studies need to include analysis of human cells as well as assays of stem/progenitor cells for establishing appropriate clinical strategies. Progress in our laboratory over the past several years has led to the development of working models of transplanted cell engraftment and proliferation in the liver. Several important mechanisms have been identified that regulate the entry of transplanted cells in the liver parenchyma and further determine whether transplanted cells will proliferate or not. Specific perturbations in the host liver that promote cell engraftment include analysis of cells in liver sinusoids, modulations in the liver microenvironment and aspects of the biological properties of transplanted cells themselves. Genotoxic injury in native hepatocytes that spare transplanted cells from injury appears to be highly effective in promoting transplanted cell proliferation. A major goal of our continuing studies in this area concerns development of novel mechanisms in liver repopulation. We propose a series of studies to further define cell engraftment in the normal and the diseased liver of animals. We will study the fate of human liver cells in immunodeficient mice to generate further animal models. We will determine whether transplanted cells will proliferate following novel ways to induce genotoxic injury in the liver of recipient animals prior to cell transplantation. We will use these principles to approach correction of metabolic and genetic disorders in animals by liver-directed cell therapy. Completion of our studies will provide new knowledge in the areas of liver repopulation, stem cell biology and cell therapy. Our studies will provide valuable preclinical information for eventual applications of cell therapy in people.